my transmissions alive!

After i drove my car in the garage, i put the trans in park and shut off the engine.I was sitting in the car for about 3 mins typing a text when suddenly the car jerked as though it was alive! keeping in mind it was in PARK and the engine was OFF...it has never done this before and i suspect it has something to do with the transmission. Has this happened to anyone before? let me know

The floor is level and no, work has not been done to the brake system. Im almost certain it was the transmission because when it jerked in made a "click" noise and when my trans shifts through its gears it will occsaionaly "click"